1. Field of the Invention
The present invention relates to an antenna device provided to a vehicle to receive signals transmitted from an artificial satellite, and more particularly, to a vehicle antenna device having a simplified structure so that the size and cost can be reduced.
2. Description of the Related Art
In recent years, a system to receive signal waves transmitted from multiple artificial satellites that orbit around the earth by a receiver and detect the present position of the receiver based on information included in the received signal waves has come into widespread use. The system is generally called GPS (Global Positioning System) in countries including Japan and the United States of America and typically uses the GPS satellites controlled by the U.S. Department of Defense, while there are similar systems such as Galileo in Europe and Glonass in the Russian Federation. Herein, the positioning system using artificial satellites, the artificial satellites for the positioning system, signal waves transmitted from the artificial satellites, and receivers receiving the signal waves will be referred to as GPS, GPS satellites, GPS signals, and GPS receivers, respectively for ease of representation.
The GPS allows the present position of a moving body to be detected highly accurately and almost in real time, and therefore the system is primarily used for measuring the present position of a moving body such as an automobile, an airplane, and a mobile telephone using a receiver provided in the moving body.
Today, GPS receivers suitable for automobiles, in other words, vehicle GPS receivers have rapidly come into widespread use. When such a GPS receiver is provided in an automobile, an antenna device for receiving a GPS signal is provided on the exterior of the automobile such as on the roof.
Also in recent years, a satellite broadcasting system in which signals including audio and video information are transmitted from a broadcasting satellite for broadcasting has been in wide use. At present, in the United States of America, audio sound information provided by such a satellite broadcasting system, so-called satellite radio broadcasting is provided by XM Satellite Radio Inc. In the satellite radio broadcasting, signals transmitted from a satellite can be received in a wide area on the earth, and therefore the broadcasting can be received and listened to not only in fixed locations such as in general households with a receiver, but also in a moving body (vehicle) such as an automobile with a receiver provided in the moving body. The latter case has attracted much attention.
An antenna device 100 as shown in FIG. 2 is an example of the vehicle antenna device to receive the above-described GPS signals or satellite broadcasting signals, in other words, signals transmitted from an artificial satellite.
As shown in FIG. 3, the conventional antenna device 100 includes an antenna module 101 including a ceramic material formed into a rectangular plate shape and antenna elements formed on both surfaces of the ceramic material, a circuit board 103 joined to one main surface of the antenna module 101 through an adhesive material 102 such as a length of double-faced adhesive tape, and a shield case 104 to shield the circuit board 103. Note that the antenna module 101 is connected with the circuit board 103 by feed pins 101a as shown in FIG. 3. The circuit board 103 is provided with electronic parts forming a low noise amplifying circuit (LNA circuit) that amplifies a signal received by the antenna module 101. The circuit board 103 is connected with an output cable 105 to extract the signal amplified by the LNA circuit to the outside (see Japanese Patent Laid-Open No. 2001-68912).
There has been a demand for smaller vehicle antenna devices in order to improve the appearance of the vehicle exterior. A so-called diversity method by which a number of antenna devices are provided to a vehicle has been suggested in order to improve the receiving sensitivity. By this method, a plurality of antenna devices must be provided to the vehicle, and the demand for smaller size, less costly antenna devices is strong.
The conventional antenna device 100 however includes a large number of parts as described above, and therefore there is a limit to the size and cost reduction.